1. Field of the Invention
The present invention relates to an optical transmitting and receiving device incorporating a light emitting device (LED) and a light receiving device into a substrate.
2. Description of the Related Art
Lately, although communication networks using optical fibers are being put into practice, so far only trunk route communication networks have been formed using optical fibers, and channels laid from trunk route networks to subscribers, that is, subscriber channels, still remain as electrical circuits. To further spread the use of optical communication networks, it is desired that all the subscriber channels also consist of optical channels. To meet this expectation a variety of research and development is actively being carried out.
The cost reduction of optical devices including a WDM filter constitutes a very important factor in the construction of an optical subscriber system using a wavelength multiplex technology called an xe2x80x9cATM-PON (Asynchronous Transfer Mode-Passive Optical Network). To reduce the cost, the mounting of a small number of compact optical devices is indispensable, and a device form in which an LED, a light receiving device and a WDM filter are hybrid-mounted on a substrate is expected to be developed. Furthermore, since the transmitting unit and receiving unit of a optical module operate asychronously in the ATM-PON system, it is necessary that the crosstalk between transmission and reception in the module is sufficiently small.
FIG. 1 explains the configuration of a conventional optical transmitting and receiving device.
As shown in FIG. 1, in the conventional optical device an LED 1004, a light receiving device 1005 and a WDM filter 1002 are encapsulated as discrete devices, and the devices are connected to an optical network and with each other using optical fibers 1001 and 1003, respectively. In the optical module with this configuration the CAN packages of the LED 1004 and the light receiving device 1005 are utilized as electrostatic shielding to suppress the crosstalk between transmission and reception signals.
Recently, although a miniature optical transmitting and receiving device hybrid-mounting an LED, a light receiving device and a WDM filter on a waveguide substrate is being developed, the application of this optical device is limited to a TCM (Time-Compression Multiplexing) transmission system for time-dividing transmitting time and receiving time. This is because the crosstalk from the transmitting unit to the receiving unit is difficult to suppress. In the transmitting unit, several tens of milliamperes of current are required to drive the LED, whereas the receiving unit requires only very little current, in the order of a microampere or less. For this reason, the current in the receiving unit is required to be in the order of 10 to 100 nA because of the crosstalk from the transmitting unit.
FIG. 2 explains how crosstalk is generated between an LED and a light receiving device in a hybrid-mounted optical transmitting and receiving device.
To simplify the description, only the minimum necessary component elements are shown in the diagram.
In an optical transmitting and receiving device hybrid-mounting an LED (laser diode: LD) 1100 and a light receiving device (photodiode: PD) 1101, a silicon dioxide film (SiO2) 1104 is formed on a silicon (Si) substrate 1106, and on the silicon dioxide film electrodes 1102 and 1103 are formed. Then, the electrodes 1102 and 1103 are connected to the LD 1100 and PD 1101, respectively. A metallic film (not shown in the diagram) is provided to ground the back of the substrate 1106. Although the silicon dioxide film 1104 is provided so that current may not flow in the electrode 1103 of the PD 1101 due to the voltage generated by the electrode 1102, current leaks to the substrate 1106 by the effect of alternating voltage applied to LD 1100 since the insulation function of the silicon dioxide film 1104 is not complete and the silicon dioxide film 1104 itself has its own capacitance. At this moment, although much of the current flows out of the substrate 1106 since the back of the substrate 1106 is grounded, part of the current reaches the electrode 1103 through the inside of the substrate 1106. Although the current reaching the electrode 1103 is small, significant noise appears on the signal generated by the PD 1101 due to the current reaching the electrode 1103 through the substrate 1106, since there is a significant difference between the current for driving the LD 1100 and the current generated by the PD 1101, as described before. Accordingly, the performance of the PD 1101 in detecting optical signals from the received light beams becomes lower because of this generated current.
In this way, as a result of the conventional configuration, significant crosstalk is generated between the transmitting side and receiving side through the substrate 1106.
The silicon dioxide film 1105 is a heat-oxidized film generated during the substrate processing, and if the back of the substrate 1106 is left unprocessed, the thickness of this silicon dioxide film 1105 will grow to approximately 2 xcexcm.
To realize a miniature optical device for an ATM-PON, it is necessary to hybrid-mount an LED, a light receiving device and a WDM filter on the same substrate, and to reduce the crosstalk between the transmitting side and the receiving side as described before. The crosstalk between the transmitting side and the receiving side is roughly classified into two groups; crosstalk due to an optical cause such as stray light, etc., and crosstalk due to an electrical cause such as free capacitance, etc.
As described before, the electrical cause is generated by a current flowing between the transmitting side and receiving side through the substrate, and this is a serious problem.
The stray light, etc. is generated by light beams emitted from the LD leaking out from an optical waveguide and generating a mode spreading over all the substrate. Accordingly, when the PD receives such stray light, it becomes impossible to accurately receive optical signals.
It is an object of the present invention to provide a miniature optical device in which transmission and reception can be simultaneously operated, by reducing the crosstalk between a hybrid-mounted LED and light receiving device.
The optical transmitting and receiving device of the present invention comprises a conduction layer formed on all or a part of the surface of a substrate, an insulation layer formed at least at the bottom of an LED mounting portion and a light receiving device mounting portion, an optical waveguide formed on the surface of the insulation layer, electric wiring patterns formed on the surface of the insulation layer, and an LED and a light receiving device connected to the electric wiring patterns so as to be optically coupled with the optical waveguide. The optical transmitting and receiving device is characterized in that the above-mentioned conduction layer is made electrically connectable to a constant potential portion.
The manufacturing method of the optical transmitting and receiving device of the present invention comprises the steps of forming a conduction layer by doping an impurity on the surface of the substrate, laminating an insulation layer on the surface of the conduction layer, providing an optical waveguide on the insulation layer and mounting an LED and a light receiving device.
The optical transmitting and receiving device in another aspect of the present invention is characterized in that in an optical transmitting and receiving device hybrid-mounting at least an LED and a light receiving device on the same substrate through the insulation layer, a conduction layer is located at least at the bottom of the above-mentioned LED and the above-mentioned light receiving device, and between the above-mentioned substrate and the above-mentioned insulation layer, and the conduction layer can be electrically connected to a constant potential portion.
The optical transmitting and receiving device in another aspect of the present invention is characterized in that in an optical transmitting and receiving device hybrid-mounting at least an LED and a light receiving device on the same substrate through the insulation layer, the above-mentioned substrate is a semiconductor substrate of one (p or n type) conduction type, and a semiconductor layer of one (n or p type) conduction type the reverse of the above-mentioned (p or n type) conduction type, forming a pn junction together with the above-mentioned semiconductor substrate, and is located at least at the bottom of the above-mentioned LED and the above-mentioned light receiving device and between the above-mentioned semiconductor substrate and the above-mentioned insulation layer, and the voltage applied between the above-mentioned LED and a light receiving device, and the back of the above-mentioned semiconductor substrate is biased the reverse of the above-mentioned pn junction.
The manufacturing method of the optical transmitting and receiving device in another aspect of the present invention is characterized in that in the manufacturing method of an optical transmitting and receiving device hybrid-mounting at least an LED and a light receiving device on the same substrate through the insulation layer, comprises a process for forming a conduction layer electrically connectable to the constant potential portion on the surface layer portion of the above-mentioned substrate and at the bottom of at least the above-mentioned LED mounting portion and the above-mentioned light receiving device mounting portion, prior to the formation of the above-mentioned insulation layer.
The manufacturing method of the optical transmitting and receiving device in another aspect of the present invention is characterized in that in the manufacturing method of an optical transmitting and receiving device hybrid-mounting at least an LED and a light receiving device on the same substrate through the insulation layer, comprises a process for forming a conduction layer of one (n or p type) conduction type the reverse of the above-mentioned (p or n type) conduction type, forming a pn junction together with the above-mentioned semiconductor substrate, on the surface layer portion of the above-mentioned substrate and at the bottom of a portion mounting at least the above-mentioned LED and the above-mentioned light receiving device, prior to the formation of the above-mentioned insulation layer using a semiconductor substrate of one (p or n type) conduction type for the above-mentioned substrate.
The platform of the optical transmitting and receiving device of the present invention comprises a conduction layer formed on all or a part of the surface of the substrate, an insulation layer formed at the bottom of portions mounting at least an LED and a light receiving device, an optical waveguide formed on the surface of the insulation layer, and an electric wiring pattern formed on the surface of the insulation layer, and is characterized in that the above-mentioned conduction layer can be electrically connected to a constant potential portion.
The manufacturing method of the platform of the optical transmitting and receiving device of the present invention comprises the steps of forming a conduction layer by doping an impurity on the surface of the substrate, laminating an insulation layer on the surface of the conduction layer, providing an optical waveguide on the surface of the insulation layer, and forming an electric wiring pattern on the surface of the insulation layer.
According to such an optical transmitting and receiving device of the present invention, since the current leaking from the LED to the substrate through the conduction layer can be eliminated, no current leaks to the light receiving device side. Accordingly, the crosstalk between the transmitting side and the receiving side can be suppressed, the transmission side and the receiving side can be simultaneously operated, and thereby a miniature hybrid-mounted optical transmitting and receiving device required to construct a subscriber system in an optical communication network using an optical circuit can be provided.